A power conversion apparatus, such as an inverter, uses a diode, referred to as an FWD (Free Wheeling Diode), and an IGBT (Insulated Gate Bipolar Transistor) which are combined into a power semiconductor device. The FWD is composed of an anode layer, which is a p-type semiconductor layer, and a cathode layer which is formed with a high-resistive semiconductor layer (an active layer, hereinafter) and an n-type semiconductor layer. At a state where a load having inductance is connected to the power conversion apparatus such as an inverter, when the IGBT is turned off, the current flows to the FWD connected in parallel to the inductance. In order to minimize power loss (steady loss) of the FWD, a forward voltage drop Vf of the FWD is required to be low. As carriers in the active layer increase, the forward voltage drop Vf is lowered more.
When the IGBT is turned on after the passage of a certain period of time, the free wheeling current flowing through the FWD decreases to turn off the FWD. In order to decrease a switching loss at the time of turn-off, a switching time is preferably short. However, in order to switch the FWD in a short time, carriers in the active layer are required to be discharged in a short time, which increases loss due to increase in current (reverse recovery current) for carrier discharge.
An MPS (Merged P-i-N/Schottky) diode is a diode of a small reverse recovery current. The MPS diode has an ohmic contact region and a Schottky contact region in its anode electrode. As the hole injection from the Schottky contact region is less, a reverse recovery current is smaller, which requires to lower the impurity concentration of the Schottky contact region. However, when the impurity concentration of the Schottky contact region is lowered at the time of turn-off, a depletion layer reaches the anode electrode, which causes a problem in that a rated breakdown voltage cannot be obtained.
In order to raise the breakdown voltage, it is desirable to restrict the spread of a depletion layer extending to the anode electrode side at the time of applying the reverse voltage. It is, for example, considered to provide a p-type stopper layer in an anode region. However, when the stopper layer is provided, the stopper layer becomes a barrier against electrons, so that carriers are charged in the active layer to increase the reverse recovery current.
As explained above, it is not easy to satisfy both of the requirements of higher breakdown voltage and smaller reverse recovery current.